A typical combustion apparatus in the prior art is so constructed that a throttle plate is provided within a heat exchanger frame to form a primary combustion chamber together with the heat exchanger frame, a flame aperture surface of a burner is arranged to face the inside the primary combustion chamber at a position opposite to an opening formed in the throttle plate, a premixed gas mixture of fuel gas and primary air is ejected into the primary combustion chamber from the flame aperture surface of the burner, and also secondary air is supplied to the primary combustion chamber from the both sides of the flame aperture surface. There is further formed a secondary combustion chamber on the downstream side of the throttle plate. The prior combustion apparatus with this construction has suffered problems as follows.
A variety of fuels have been used for the forced flue-type combustion apparatus such as, for example, fuel having a less degree of diffusion including gasified liquid fuel such as, kerosene and LPG (Liquid petroleum gas comprising butane and propane), fuel that has low velocity of combustion including natural gas, and fuel that has high velocity of combustion including city gas containing about 60% of hydrogen. These fuels have their own specific characteristics different from each other such that the fuel having a less degree of diffusion is liable to a reddish flame phenomenon, the fuel having low velocity of combustion is liable to a lift phenomenon of combustion flames, and the fuel having high velocity of combustion is liable to a flash-back phenomenon. When attempting to use a variety of gas fuel and gasified liquid fuel such as kerosene in a single combustion apparatus, it is required to reduce a flow rate of primary air to the combustion apparatus so as to lower the velocity of combustion and also to narrow a width of the flame aperture formed in the flame aperture surface of the burner, for the purpose of preventing back fire in the case of burning the fuel with high velocity of combustion. The low flow rate of primary air causes soot to be increasingly generated and a period of time necessary for a burning reaction to be prolonged, so that a relatively large space is required to complete the combustion reaction and a flame is largely elongated in a direction toward the downstream. stream. In general, combustion gas is converted into CO.sub.2 gas in the perfect combustion state after having passed through the process of an oxidation reaction of CO on the way. This oxidation of CO requires a sufficiently high ambient temperature under which the gas containing CO is to be reactioned, an oxidizing agent necessary for oxidation, and a reaction space. If not-yet-reactioned gas flows into fins of the heat exchanger frame, the ambient temperature would be reduced to 200.degree.-300.degree. C. and this causes the oxidation reaction of CO to be stopped. Thus, the prior apparatus has been accompanied by such a disadvantage that an increased length of the flame makes it necessary to sufficiently enlarge a height of the secondary combustion chamber, and to allow combustion gas to perfectly react with air and thereafter to flow into the fins of the heat exchanger, thereby resulting in an increase in a height of the secondary combustion chamber. Furthermore, since a width of the flame is fairly narrower than that of a flow channel in the secondary combustion chamber, there occurs a large ineffective area on the both side of the flame within the secondary combustion chamber so that the combustion apparatus includes a dead space therein. It is also required to supply a large amount of secondary air from the under side of the throttle plate, so that a flow rate of the air passing through the opening in the throttle plate becomes large. This causes the flame to be unstable and a pressure loss of the premixed gas mixture ejected from the burner to be increased, thus resulting in such a disadvantage that a fan for use in supplying the premixed gas mixture requires an increased amount of power consumption. When passing through the opening in the throttle plate, the secondary air diffuses into a reaction portion of combustion gas at a low degree and hence most of the secondary air not diffused remains in the aforesaid ineffective area. This also entails a disadvantage that such secondary air will be discharged out to the exterior without effectively contributing to the reaction of CO contained in combustion gas.